Method for enhancing stability of oil based drilling fluids at high temperatures

ABSTRACT

Methods for enhancing the stability of ester based drilling fluids for drilling, running casing in, and/or cementing a borehole in a subterranean formation are disclosed, as well as improved ester based drilling fluids for use at high temperatures. The advantages of the invention are realized by inhibiting hydrolysis of the esters in the drilling fluid by adding a monomeric or polymeric carbodiimide hydrolysis inhibitor to the drilling fluid.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to compositions and methods for drilling,cementing and casing boreholes in subterranean formations, particularlyhydrocarbon bearing formations. More particularly, the present inventionrelates to oil or synthetic fluid based drilling fluids and fluidscomprising invert emulsions, such as fluids using esters for example,which combine high ecological compatibility with good stability andperformance properties.

2. Description of Relevant Art

A drilling fluid or mud is a specially designed fluid that is circulatedthrough a wellbore as the wellbore is being drilled to facilitate thedrilling operation. The various functions of a drilling fluid includeremoving drill cuttings from the wellbore, cooling and lubricating thedrill bit, aiding in support of the drill pipe and drill bit, andproviding a hydrostatic head to maintain the integrity of the wellborewalls and prevent well blowouts. Specific drilling fluid systems areselected to optimize a drilling operation in accordance with thecharacteristics of a particular geological formation.

Oil or synthetic fluid-based muds are normally used to drill swelling orsloughing shales, salt, gypsum, anhydrite or other evaporate formations,hydrogen sulfide-containing formations, and hot or high temperature(greater than about 300 degrees Fahrenheit (“° F.”)) holes, but may beused in other holes penetrating a subterranean formation as well. Unlessindicated otherwise, the terms “oil mud” or “oil-based mud” or “drillingfluid” shall be understood to include synthetic oils or other syntheticfluids as well as natural or traditional oils, and such oils shall beunderstood to comprise invert emulsions.

Oil-based muds used in drilling typically comprise: a base oil (orsynthetic fluid) comprising the external phase of an invert emulsion; asaline, aqueous solution (typically a solution comprising about 30%calcium chloride) comprising the internal phase of the invert emulsion;emulsifiers at the interface of the internal and external phases; andother agents or additives for suspension, weight or density,oil-wetting, fluid loss or filtration control, and rheology control.

An oil-based or invert emulsion-based drilling fluid may commonlycomprise between about 50:50 to about 95:5 by volume oil phase to waterphase. An all oil mud simply comprises 100% liquid phase oil by volume;that is, there is no aqueous internal phase.

Invert emulsion-based muds or drilling fluids (also called invertdrilling muds or invert muds or fluids) comprise a key segment of thedrilling fluids industry. However, increasingly invert emulsion-baseddrilling fluids have been subjected to greater environmentalrestrictions and performance and cost demands. While ester based invertemulsion drilling fluids satisfy many environmental concerns, they tendto lose effectiveness or utility at high oilfield temperatures. There isconsequently an increasing need and industry-wide interest in newdrilling fluids and methods that provide improved performance whilestill affording environmental and economical acceptance.

SUMMARY OF THE INVENTION

The present invention provides improved methods of drilling wellbores insubterranean formations employing oil-based muds or drilling fluids. Asused herein, the term “drilling” or “drilling wellbores” shall beunderstood in the broader sense of drilling operations, which includerunning casing and cementing as well as drilling, unless specificallyindicated otherwise. More particularly, the present invention provides amethod for improving the stability of oil-based drilling fluids, mostparticularly at high temperatures. The present invention also providesoil based drilling fluids having improved stability at such hightemperatures.

Any drilling fluid comprising a hydrolysable component may benefit fromthe present invention. Esters and amides are particularly susceptible tohydrolysis and thus the invention is particularly advantageous withinvert emulsion drilling fluids comprising esters and/or amides,particularly as the base oil for the drilling fluids but also asadditives or components in the drilling fluids, such as for example,emulsifiers and fluid loss control additives. The benefits of theinvention are obtained by inhibiting hydrolysis of esters and/or amidesin or comprising the drilling fluid. Such benefits are particularlyrealized at higher temperatures, that is, at temperatures of 275° F. orhigher and extending to temperatures as high as about 450° F. or more,as may be encountered in drilling boreholes in subterranean formationsfor recovery of hydrocarbons.

According to the method of the invention, a carbodiimide, preferably apolycarbodiimide, is added to the drilling fluid as a hydrolysisinhibitor. A 5 lb/bbl loading amount will typically be effective forpurposes of the invention, although other amounts may be successfullyused with experimentation. Drilling fluids of the invention comprise anester oil or invert emulsion base and/or ester or amide additives, suchas, for example, emulsifiers and fluid loss control additives, and acarbodiimide, or preferably a polycarbodiimide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph comparing the amount of alcohol produced, indicatinghydrolysis, in samples containing ester base and water, with and withoutpolycarbodiimide additives, after heating at 370° F. for 16 hours.

FIG. 2 is a graph comparing the amount of alcohol produced, indicatinghydrolysis, in samples containing ester base and water, with and withoutpolycarbodiimide additives, after heating at 370° F. for periods of 16hours, 40 hours, 64 hours and 88 hours.

FIG. 3 is a graph comparing the amount of alcohol produced, indicatinghydrolysis, in samples containing ester base, water, and lime, with andwithout polycarbodiimide additives, after heating at 370° F. for 16hours.

FIG. 4 is a graph comparing the amount of alcohol produced, indicatinghydrolysis, in samples of an ester based invert emulsion drilling fluid,with and without polycarbodiimide additives, after hot rolling at 150°F. for 16 hours and then at 370° F. for 16 hours.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Ester based invert emulsion drilling fluids are favored as oil baseddrilling fluids for environmental acceptability and regulatorycompliance. However, in prior art fluids, the ester can be vulnerable tohydrolysis, particularly at high temperatures that can be encountered indrilling subterranean formations for the recovery of hydrocarbons. Suchhydrolysis can result in breakdown of the fluid, which ultimately oreventually renders the fluid unusable as a drilling fluid. Consequently,the use of ester based drilling fluids is less common at hightemperatures, which typically increase the rate of ester hydrolysis.

The addition of lime to a drilling fluid, as has been common practice inthe oilfield, can have the negative side-effect of increasing the amountof hydrolysis of esters in prior art drilling fluids. Lime provides agreater concentration of nucleophilic hydroxide ion to the fluid, whichcan escalate hydrolysis. Consequently, lime is usually avoided in esterbased drilling fluids.

The present invention provides a method for enhancing the stability ofester-based drilling fluids and drilling fluid additives, even at hightemperatures, and even in the presence of lime or other sources ofhydroxide ion. This enhancement is accomplished by reducing the amountof hydrolysis that occurs in oil-based drilling fluid systems.Hydrolysis inhibition according to the present invention allows forgreater application of ester based drilling fluids in higher temperatureenvironments with greater retention of desirable fluid properties.

In the method of the present invention, enhanced stability of esterbased invert emulsion drilling fluids is obtained by adding to orincluding in the drilling fluid a carbodiimide, preferably apolycarbodiimide. The carbodiimide carbon is believed to be morereactive than the carbonyl carbon of an ester and hence the reaction ofthe carbodiimide with water or hydroxide (hydrolysis) is expected to bemore facile than hydrolysis of the ester of the drilling fluid.Consequently, the reaction of the carbodiimide with the water orhydroxide prevents or inhibits the carbonyl carbon of the ester fromreacting with the water or hydroxide that might otherwise hydrolyze theester, resulting in the ester not becoming hydrolyzed. This isparticularly significant, and surprising, in drilling fluids, andespecially invert emulsion drilling fluids, because such fluids alreadycontain water as part of their composition. The carbodiimide isnevertheless able to maintain effectiveness in enhancing the stabilityof the fluid over time, as for example for a time sufficient to drillone or more wells or subterranean boreholes. That is, the carbodiimideis not immediately “used up” or diluted to ineffectiveness uponplacement into the fluid.

The effectiveness of the invention is demonstrated in the followingexperiments.

Experiment 1:

Since hydrolyzation of an ester causes part of the ester molecule to belost as an alcohol, the effectiveness of hydrolysis inhibitors can bemeasured by monitoring the production of alcohol that is formed in esterhydrolysis. In the case of a state-of-the-art, ester-based, invertemulsion fluid sold by Halliburton Energy Services, Inc., in Houston,Tex. and Duncan, Okla. under the tradename ACCOLADE® drilling fluid, theamount of alcohol produced can be monitored by gas chromatography/massspectrometry. Thus, samples containing ACCOLADE® base fluid, calciumchloride brine, and an emulsifier, were formulated with and without 5lb/bbl loading of one of two commercial polycarbodiimides: the viscousliquid STABAXOL® P200 polycarbodiimide (Sample A) and the powderSTABAXOL® P polycarbodiimide (Sample B), both available from RheinChemie Rheinau GmbH, Mannheim, Deutschlandas (Germany). The samples werestatic aged at 370° F. for 16 hours and then tested for alcohol content,as indicated in Table 1 below.

TABLE 1 Additive wt. % alcohol ppm alcohol Polycarbodiimide Sample A0.041 409 Polycarbodiimide Sample B 0.053 529 None (Control) 0.61 6133

The data is also graphed in FIG. 1. The data indicates that a greaterthan 90% reduction in the amount of alcohol produced was observed in thepresence of the polycarbodiimides. This reduction indicates a reductionin the amount of hydrolysis of the ester in the ACCOLADE® base fluid.

Experiment 2:

Samples of the same compositions of Experiment 1 were static aged at370° F. for 16 hours, 40 hours, 64, hours and 88 hours, and tested foralcohol content after each elapsed aging period, as shown in Table 2below.

TABLE 2 Additive wt. % alcohol ppm alcohol Polycarbodiimide Sample A0.06 570 Static Aged 16 hours Polycarbodiimide Sample B 0.16 1588 StaticAged 16 hours None (Control) 0.59 5876 Static Aged 16 hoursPolycarbodiimide Sample A 0.13 1333 Static Aged 40 hoursPolycarbodiimide Sample B 0.33 3309 Static Aged 40 hours None (Control)1.00 10005 Static Aged 40 hours Polycarbodiimide Sample A 0.34 3365Static Aged 64 hours Polycarbodiimide Sample B 0.74 7373 Static Aged 64hours None (Control) 1.25 12470 Static Aged 64 hours PolycarbodiimideSample A 0.74 7398 Static Aged 88 hours Polycarbodiimide Sample B 1.3413427 Static Aged 88 hours None (Control) 1.37 13749 Static Aged 88hours

The data is also graphed in FIG. 2. The data indicates that after 64hours, both polycarbodiimide additives still reduced the alcoholconcentration, and after 88 hours, Additive A still showed the abilityto reduce the amount of ester hydrolysis that occurred. The reasonAdditive A showed superior performance to Additive B is unclear,although the liquid nature of the material may have allowed the materialto be more fully solubilized in the solids-free fluids tested in thisexperiment leading to greater efficacy.

Experiment 3:

Samples containing ACCOLADE® base, calcium chloride brine, and anemulsifier, were formulated with and without 5 lb/bbl loading of one oftwo commercial polycarbodiimides, STABAXOL® P200 and STABAXOL® Ppolycarbodiimides, available from Rhein Chemie Rheinau GmbH, Mannheim,Deutschlandas, as in Experiments 1 and 2, except to each of the samplesfor this Experiment 3 was added lime (3 lb/bbl). The samples were staticaged at 370° F. for 16 hours and tested for alcohol content, as shown inTable 3.

TABLE 3 Additive wt. % alcohol ppm alcohol Polycarbodiimide Sample A0.64 6434 Polycarbodiimide Sample B 0.56 5556 None (Control) 0.98 9804

The data is also graphed in FIG. 3. This data shows that a significantreduction in hydrolysis is obtained with the polycarbodiimide additives,even in the presence of lime, and even though the addition of lime to adrilling fluid typically increases the amount of hydrolysis due to thepresence of greater concentrations of the more nucleophilic hydroxideion. The percent reductions in hydrolysis in the samples with thepolycarbodiimide additives when compared to the control were 34.4% inthe case of polycarbodiimide Sample A and 43.3% in the case ofpolycarbodiimide Sample B.

Experiment 4:

Samples containing ACCOLADE® invert emulsion drilling fluid wereformulated with and without 5 lb/bbl loading of one of two commercialpolycarbodiimides, STABAXOL® P200 and STABAXOL® P polycarbodiimides,available from Rhein Chemie Rheinau GmbH, Mannheim, Deutschlandas, asindicated in Table 4.

TABLE 4 Polycarbodiimide Sample A B None (Control) ACCOLADE ® base,0.519 0.519 0.519 bbl 250,000 ppm CaCl₂ 0.0676 0.0676 0.0676 brine, bblLE SUPERMUL ® 10 10 10 polyaminated fatty acid, lb FACTANT ® 1 1 1highly concentrated tall oil derivative, lb ADAPTA ® 3 3 3methylstyrene/acrylate copolymer for HPHT filtration control, lb RevDust, lb 20 20 20 Barite, lb 497 497 497 Polycarbodiimide 5 0 0 SampleA, lb Polycarbodiimide 0 5 0 Sample B, lb All trademarked products areavailable from Halliburton Energy Services, Inc. in Houston, Texas andDuncan, Oklahoma.

The samples were hot rolled at 150° F. for 16 hours and then were testedfor alcohol concentration. The samples were hot rolled further at 370°F. for 16 additional hours and tested for alcohol, as shown in Table 5.

TABLE 5 Additive wt. % alcohol ppm alcohol Polycarbodiimide Sample A0.04 416 hot rolled at 150° F. Polycarbodiimide Sample B 0.03 277 hotrolled at 150° F. None (Control) 0.06 626 hot rolled at 150° F.Polycarbodiimide Sample A 0.71 7129 hot rolled at 370° F.Polycarbodiimide Sample B 0.09 871 hot rolled at 370° F. None (Control)0.78 7761 hot rolled at 370° F.

This data is also graphed in FIG. 4. The data shows that the percentreduction in hydrolysis with the polycarbodiimide additive Sample B,when compared to the control, was 89% over the control after aging at370° F. Sample A did not perform as well in the whole mud, possiblybecause the material was wetted onto solids that were present in thefluid.

Although the experiments were conducted with an ACCOLADE® ester base andan ACCOLADE® invert emulsion drilling fluid, benefits of the inventionmay be seen with other ester based drilling fluids. Some other examplesof commercial ester based invert emulsion drilling fluids include,without limitation, PETROFREE®, PETROFREE® LV, and PETROFREE® F drillingfluids available from Halliburton Energy Services, Inc. in Houston, Tex.Still further examples are taught in U.S. Pat. No. 7,485,602, issuedFeb. 3, 2009, to Jeff Kirsner, et al, and in U.S. Pat. No. 7,456,135,issued Nov. 25, 2008 to Jeff Kirsner, et al., among many other examples.Also, drilling fluids having an amide base or other base vulnerable tohydrolysis may benefit from addition of a carbodiimide orpolycarbodiimide as a hydrolysis inhibitor.

Further, the invert emulsion drilling fluids of the invention or for usein the present invention have added to them or mixed with their invertemulsion base, other fluids or materials needed to comprise completedrilling fluids. Such materials may include, for example: additives toreduce or control temperature rheology or to provide thinning, forexample, additives having the tradenames COLDTROL®, ATC®, and OMC2™;additives for enhancing viscosity, for example, an additive having thetradename RHEMOD L™; additives for providing temporary increasedviscosity for shipping (transport to the well site) and for use insweeps, for example, an additive having the tradename TEMPERUS™(modified fatty acid); additives for filtration control, for example, anadditive having the tradename ADAPTA® (methylstyrene/acrylatecopolymer); additives for high temperature high pressure control (HTHP)and emulsion stability, for example, an additive having the tradenameFACTAN™ (highly concentrated tall oil derivative); additives foremulsification, for example, an additive having the tradename LESUPERMUL™ (polyaminated fatty acid); and additives for thinning, forexample additives having the tradenames COLDTROL® (alcohol derivative),OMC2™ (oligomeric fatty acid), and ATC® (modified fatty acid ester) andexample additives taught in U.S. Pat. No. 7,435,706, issued Oct. 14,2008 to Heinz Mueller, et al. and in U.S. patent application Ser. No.10/432,786, filed Dec. 10, 2003 of Heinz Mueller, et al., both entitled“Thinners for Invert Emulsions.” All of the aforementioned trademarkedproducts are available from Halliburton Energy Services, Inc. inHouston, Tex., U.S.A. Additionally, the fluids may comprise an aqueoussolution containing a water activity lowering compound, composition ormaterial, comprising the internal phase of the invert emulsion. Suchsolution may be a saline solution comprising calcium chloride (typicallyabout 25% to about 30%, depending on the subterranean formation watersalinity or activity), although other salts or water activity loweringmaterials known in the art may alternatively or additionally be used.

Many other examples of drilling fluid components will be readily knownto those of skill in the art. Also known is that such components mayinclude an ester or an amide, known to be vulnerable at least at hightemperatures to hydrolysis. For example, LE SUPERMUL®, EZ® MUL, EZ MUL®NT, EZ MUL® NS, DRILTREAT®, INVERMUL®, INVERMUL® NT and ADAPTA®additives are emulsifiers and fluid loss control additives that containesters or amides. Adding a carbodiimide, preferably a polycarbodiimide,to the drilling fluid according to the present invention enhances thestability of such additives, and consequently enhances the stability ofthe drilling fluid overall. That is, the carbodiimide inhibitshydrolysis of the ester or amide in such additives. Thus advantages maybe realized by adding a carbodiimide or polycarbodiimide to a drillingfluid containing ester or amide additives even when the base oil of thedrilling fluid may not comprise an ester or amide.

The exact formulations of drilling fluids used in drilling boreholesvary with the particular requirements of the subterranean formation. Theabove listed additives, however, provide some examples. A preferredexample drilling fluid of the present invention may have the formulationused in Experiment 4 discussed above. A couple of other preferredexample drilling fluids of the present invention have the followingexample formulations set forth in Table 6 below.

TABLE 6 Example Formulations ACCOLADE ® PETROFREE ® SF Invert EmulsionInvert Emulsion Fluids and Compounds Drilling Fluid Drilling FluidACCOLADE ® Base (bbl) 0.590 — PETROFREE ® SF BASE — 0.568 (bbl) LEMUL ™¹ (lb.) — 4 LE SUPERMUL ™² (lb.) 10 6 Lime (lb.) 1 4 DURATONE ® HT³(lb.) — 4 Freshwater (bbl) 0.263 0.254 ADAPTA ®⁴ (lb.) 2 — RHEMOD L ™⁵(lb.) 1 — GELTONE ® II⁶ (lb.) — 5 VIS-PLUS ®⁷ (lb.) — 1.5 BAROID ®⁸(lb.) 138 138 Calcium chloride (lb.) 32 31 DEEP-TREAT ®⁹ (lb.) — 2Polycarbodiimide (lb.) 5 5 ¹Blend of oxidized tall oil and polyaminatedfatty acid emulsion stabilizer. ²Polyaminated fatty acid emulsifier.³Organophilic leonardite filtration control agent. ⁴Copolymer HTHPfiltration control agent for non-aqueous systems. ⁵Modified fatty acidsuspension agent/viscosifier. ⁶Organophilic clay viscosifier.⁷Carboxylic acid suspension agent. ⁸Ground barium sulfate weightingagent. ⁹Sulfonate sodium salt wetting agent/thinner.

All trademarked products in Table 6 are available from HalliburtonEnergy Services, Inc. in Houston, Tex., including: LE MUL™ emulsionstabilizer (a blend of oxidized tall oil and polyaminated fatty acid);LE SUPERMUL™ emulsifier (polyaminated fatty acid); DURATONE® HTfiltration control agent (organophilic leonardite); ADAPTA® filtrationcontrol agent (methylstyrene/acrylate copolymer particularly suited forproviding HPHT filtration control in non-aqueous fluid systems); RHEMODL™ suspension agent/viscosifier (modified fatty acid); GELTONE® IIviscosifier (organophilic clay); VIS-PLUS® suspension agent (carboxylicacid); BAROID® weighting agent (ground barium sulfate); and DEEP-TREAT®wetting agent/thinner (sulfonate sodium salt).

Examples of preferred commercially available polycarbodiimides useful inthe formulations set forth in Table 6 include, without limitation,STABAXOL® I, STABAXOL® P, and STABAXOL® P200 polycarbodiimides, allavailable from Rhein Chemie Rheinau GmbH, Mannheim, Deutschlandas(Germany).

The preferred formula for carbodiimides for use in the present inventionis as follows:

where R and R′ are the same or different and are selected from the groupconsisting of aromatic groups and straight chain, branched, and cyclicalkyl groups, alkenyl groups, and alkynl groups. Alkyl and aromaticgroups are more preferred than alkynl or alkenyl groups. Aromaticpolycarbodiimides are preferred over straight chain, branched, or cyclicpolycarbodiimides, although straight chain, branched, or cyclicpolycarbodiimides can be used in the invention. Polymeric carbodiimidesare preferred over carbodiimides. Preferably R has C₁ to C₃₀ carbonatoms, and generally polycarbodiimides having a molecular weight in therange of about 500 to about 10,000 g/mol will be preferred for use inthe present invention.

Further carbodiimides believed to have utility in the present inventionare described in U.S. Pat. No. 6,498,225 and in U.S. Pat. No. 5,360,888,for example. U.S. Pat. No. 6,498,225 specifically teachespolycarbodiimide-based copolymers having the general formula:

X-[-(A)_(m)-(B)_(n)-]_(o)—X

where:

X is identical or different and is selected from the group consisting of—NHCO—R′″, —NHCONH—R′″, —NHCOO—R′″, —NHCOS—R′″, —COO—R′″, —O—R′″, —NR₂,—S—R′″, —OH, —S—H, —NH₂, —NHR′″, and —NCO, and wherein the group R′″denotes an alkyl, cyclcoalkyl, aralkyl or aryl radical containing 1 to30 carbon atoms;

m, n are independent of one another and are each an integer from 1 to1000;

o is an integer from 1 to 500;

A is selected from the group consisting of the carbodiimides orpolycarbodiimides of the formula: —(—N═C═N—Y—)—where Y is selected fromthe group consisting of ortho- or bisortho-substituted aromatics,aralkylenes in which the carbon atom linked to the carbodiimide group issubstituted by C₁ to C₁₄-alkyl groups, and cycloalkylenes in which thecarbon atom linked to the carbodiimide group is substituted by C₁ toC₁₄-alkyl groups; and

B is selected from the group consisting of (poly)diioles,(poly)diamines, (poly)dimercaptans, (poly)aminoalcohols,(poly)aminomercaptans and (poly)mercaptoalcohols. U.S. Pat. No.5,360,888 teaches polycarbodiimides that are reaction products ofsubstituted aromatic isocyanates, such as: 2,6-diisopropylphenylisocyanate; 1,3,5-triisopropyl-2,4,-diisocyanatobenzene;naphthalene-1,5-diisocyanate; 2,4-diissocyanato-3,5-dimethyltoluene;4,4′-methylenebis(2,6-diethylphenyl isocyanate);4,4′-methylenebis(2-ethyl-6-methylphenyl isocyanate);4,4′-methylenebis(2-isopropyl-6-methylphenyl isocyanate);4,4′-methylenebis(2,6-diisopropylphenyl isocyanate); and4,4′-methylenebis(2-ethyl-6-methylcyclohexyl isocyanate).

As indicated above, the advantages of the invention may be obtained byemploying a drilling fluid of the invention, comprising or containing anester or amide, in drilling operations, wherein the drilling fluid hasadded thereto a carbodiimide, preferably a polycarbodiimide. Thedrilling operations—whether drilling a vertical or directional orhorizontal borehole, conducting a sweep, or running casing andcementing—may be conducted as known to those skilled in the art withother drilling fluids. That is, a drilling fluid of the invention isprepared or obtained and circulated through a wellbore as the wellboreis being drilled (or swept or cemented and cased) to facilitate thedrilling operation. The drilling fluid removes drill cuttings from thewellbore, cools and lubricates the drill bit, aids in support of thedrill pipe and drill bit, and provides a hydrostatic head to maintainthe integrity of the wellbore walls and prevent well blowouts. Thespecific formulation of the drilling fluid in accordance with thepresent invention is optimized for the particular drilling operation andfor the particular subterranean formation characteristics and conditions(such as temperatures). For example, the fluid is weighted asappropriate for the formation pressures and thinned as appropriate forthe formation temperatures. Further, the fluids of the invention may berecycled during a drilling operation such that fluids circulated in awellbore may be recirculated in the wellbore after returning to thesurface for removal of drill cuttings for example. The drilling fluid ofthe invention may even be selected for use in a drilling operation tocomply with environmental regulations governing drilling operations in aparticular subterranean formation.

The foregoing description of the invention is intended to be adescription of preferred embodiments. Various changes in the details ofthe described fluids and methods of use can be made without departingfrom the intended scope of this invention as defined by the appendedclaims.

1. A method for drilling a borehole in a subterranean formation for therecovery of hydrocarbons, comprising the steps of: providing or using anoil-based drilling fluid comprising a carbodiimide; and drilling theborehole in the subterranean formation.
 2. The method of claim 1 whereinthe drilling in the subterranean formation includes temperatures in therange of the range of about 275° F. to about 450° F.
 3. The method ofclaim 1 wherein the drilling fluid comprises an ester based invertemulsion.
 4. The method of claim 1 wherein the carbodiimide has theformula:

where R and R′ are the same or different and are selected from the groupconsisting of aromatic groups and straight chain, branched, and cyclicalkyl groups, alkenyl groups, and alkynl groups.
 5. The method of claim4 wherein R has C₁ to C₃₀ carbon atoms.
 6. The method of claim 4 whereinR′ has C₁ to C₃₀ carbon atoms.
 7. The method of claim 1 wherein thecarbodiimide is a polymer.
 8. The method of claim 1 wherein thecarbodiimide is monomeric.
 9. The method of claim 1 wherein thecarbodiimide is a hydrolysis inhibitor.
 10. The method of claim 1wherein the carbodiimide has a molecular weight in the range of about500-10,000 g/mol.
 11. The method of claim 1 wherein the drilling fluidcomprises an additive comprising an ester.
 12. The method of claim 11wherein the additive is an emulsifier.
 13. The method of claim 1 whereinthe drilling fluid comprises an additive comprising an amide.
 14. Themethod of claim 1 wherein the drilling fluid comprises water.
 15. Adrilling fluid for drilling a borehole in a subterranean formationcomprising: a continuous phase; an internal phase; an emulsifier; afiltration control agent; a weighting agent; and a carbodiimide.
 16. Thedrilling fluid of claim 15 wherein the continuous phase comprises anester based invert emulsion.
 17. The drilling fluid of claim 15 whereinthe emulsifier comprises an ester or an amide.
 18. The drilling fluid ofclaim 15 wherein the filtration control agent comprises an ester or anamide.
 19. The drilling fluid of claim 15 wherein the carbodiimide is apolycarbodiimide, having a molecular weight in the range of 500 to about10,000.
 20. The drilling fluid of claim 15 wherein the wherein thecarbodiimide has the formula:

where R and R′ are the same or different and are selected from the groupconsisting of aromatic groups and straight chain, branched, and cyclicalkyl groups, alkenyl groups, and alkynl groups.